Internet Engineering Task Force (IETF) A. Durand
Request for Comments: 6333 Juniper Networks
Category: Standards Track R. Droms
ISSN: 2070-1721 Cisco
J. Woodyatt
Apple
Y. Lee
Comcast
August 2011
Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion
Abstract
This document revisits the dual-stack model and introduces the Dual-
Stack Lite technology aimed at better aligning the costs and benefits
of deploying IPv6 in service provider networks. Dual-Stack Lite
enables a broadband service provider to share IPv4 addresses among
customers by combining two well-known technologies: IP in IP (IPv4-
in-IPv6) and Network Address Translation (NAT).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6333.
Durand, et al. Standards Track [Page 1]
RFC 6333 Dual-Stack Lite August 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Requirements Language ...........................................4
3. Terminology .....................................................4
4. Deployment Scenarios ............................................4
4.1. Access Model ...............................................4
4.2. CPE ........................................................5
4.3. Directly Connected Device ..................................6
5. B4 Element ......................................................7
5.1. Definition .................................................7
5.2. Encapsulation ..............................................7
5.3. Fragmentation and Reassembly ...............................7
5.4. AFTR Discovery .............................................7
5.5. DNS ........................................................8
5.6. Interface Initialization ...................................8
5.7. Well-Known IPv4 Address ....................................8
6. AFTR Element ....................................................9
6.1. Definition .................................................9
6.2. Encapsulation ..............................................9
6.3. Fragmentation and Reassembly ...............................9
6.4. DNS .......................................................10
6.5. Well-Known IPv4 Address ...................................10
6.6. Extended Binding Table ....................................10
7. Network Considerations .........................................10
7.1. Tunneling .................................................10
7.2. Multicast Considerations ..................................10
8. NAT Considerations .............................................11
8.1. NAT Pool ..................................................11
8.2. NAT Conformance ...........................................11
8.3. Application Level Gateways (ALGs) .........................11
8.4. Sharing Global IPv4 Addresses .............................11
8.5. Port Forwarding / Keep Alive ..............................11
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9. Acknowledgements ...............................................12
10. IANA Considerations ...........................................12
11. Security Considerations .......................................12
12. References ....................................................13
12.1. Normative References .....................................13
12.2. Informative References ...................................14
Appendix A. Deployment Considerations .............................16
A.1. AFTR Service Distribution and Horizontal Scaling ...........16
A.2. Horizontal Scaling .........................................16
A.3. High Availability ..........................................16
A.4. Logging ....................................................16
Appendix B. Examples ..............................................17
B.1. Gateway-Based Architecture .................................17
B.1.1. Example Message Flow ...................................19
B.1.2. Translation Details ....................................23
B.2. Host-Based Architecture ....................................24
B.2.1. Example Message Flow ...................................27
B.2.2. Translation Details ....................................31
1. Introduction
The common thinking for more than 10 years has been that the
transition to IPv6 will be based solely on the dual-stack model and
that most things would be converted this way before we ran out of
IPv4. However, this has not happened. The IANA free pool of IPv4
addresses has now been depleted, well before sufficient IPv6
deployment had taken place. As a result, many IPv4 services have to
continue to be provided even under severely limited address space.
This document specifies the Dual-Stack Lite technology, which is
aimed at better aligning the costs and benefits in service provider
networks. Dual-Stack Lite will enable both continued support for
IPv4 services and incentives for the deployment of IPv6. It also
de-couples IPv6 deployment in the service provider network from the
rest of the Internet, making incremental deployment easier.
Dual-Stack Lite enables a broadband service provider to share IPv4
addresses among customers by combining two well-known technologies:
IP in IP (IPv4-in-IPv6) and Network Address Translation (NAT).
This document makes a distinction between a dual-stack-capable and a
dual-stack-provisioned device. The former is a device that has code
that implements both IPv4 and IPv6, from the network layer to the
applications. The latter is a similar device that has been
provisioned with both an IPv4 and an IPv6 address on its
interface(s). This document will also further refine this notion by
distinguishing between interfaces provisioned directly by the service
provider from those provisioned by the customer.
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Pure IPv6-only devices (i.e., devices that do not include an IPv4
stack) are outside of the scope of this document.
This document will first present some deployment scenarios and then
define the behavior of the two elements of the Dual-Stack Lite
technology: the Basic Bridging BroadBand (B4) element and the Address
Family Transition Router (AFTR) element. It will then go into
networking and NAT-ing considerations.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Terminology
The technology described in this document is known as Dual-Stack
Lite. The abbreviation "DS-Lite" will be used throughout this text.
This document also introduces two new terms: the DS-Lite Basic
Bridging BroadBand (B4) element and the DS-Lite Address Family
Transition Router (AFTR) element.
Dual-stack is defined in [RFC4213].
NAT-related terminology is defined in [RFC4787].
CPE stands for Customer Premise Equipment. This is the layer 3
device in the customer premise that is connected to the service
provider network. That device is often a home gateway. However,
sometimes computers are directly attached to the service provider
network. In such cases, such computers can be viewed as CPEs as
well.
4. Deployment Scenarios
4.1. Access Model
Instead of relying on a cascade of NATs, the Dual-Stack Lite model is
built on IPv4-in-IPv6 tunnels to cross the network to reach a
carrier-grade IPv4-IPv4 NAT (the AFTR), where customers will share
IPv4 addresses. There are a number of benefits to this approach:
o This technology decouples the deployment of IPv6 in the service
provider network (up to the customer premise equipment or CPE)
from the deployment of IPv6 in the global Internet and in customer
applications and devices.
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o The management of the service provider access networks is
simplified by leveraging the large IPv6 address space.
Overlapping private IPv4 address spaces are not required to
support very large customer bases.
o As tunnels can terminate anywhere in the service provider network,
this architecture lends itself to horizontal scaling and provides
some flexibility to adapt to changing traffic load. More
discussion of horizontal scaling can be found in Appendix A.
o Tunnels provide a direct connection between B4 and the AFTR. This
can be leveraged to enable customers and their applications to
control how the NAT function of the AFTR is performed.
A key characteristic of this approach is that communications between
end-nodes stay within their address family. IPv6 sources only
communicate with IPv6 destinations, and IPv4 sources only communicate
with IPv4 destinations. There is no protocol family translation
involved in this approach. This simplifies greatly the task of
applications that may carry literal IP addresses in their payloads.
4.2. CPE
This section describes home Local Area networks characterized by the
presence of a home gateway, or CPE, provisioned only with IPv6 by the
service provider.
A DS-Lite CPE is an IPv6-aware CPE with a B4 interface implemented in
the WAN interface.
A DS-Lite CPE SHOULD NOT operate a NAT function between an internal
interface and a B4 interface, as the NAT function will be performed
by the AFTR in the service provider's network. This will avoid
accidentally operating in a double-NAT environment.
However, it SHOULD operate its own DHCP(v4) server handing out
[RFC1918] address space (e.g., 192.168.0.0/16) to hosts in the home.
It SHOULD advertise itself as the default IPv4 router to those home
hosts. It SHOULD also advertise itself as a DNS server in the DHCP
Option 6 (DNS Server). Additionally, it SHOULD operate a DNS proxy
to accept DNS IPv4 requests from home hosts and send them using IPv6
to the service provider DNS servers, as described in Section 5.5.
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Note: If an IPv4 home host decides to use another IPv4 DNS server,
the DS-Lite CPE will forward those DNS requests via the B4 interface,
the same way it forwards any regular IPv4 packets. However, each DNS
request will create a binding in the AFTR. A large number of DNS
requests may have a direct impact on the AFTR's NAT table
utilization.
IPv6-capable devices directly reach the IPv6 Internet. Packets
simply follow IPv6 routing, they do not go through the tunnel, and
they are not subject to any translation. It is expected that most
IPv6-capable devices will also be IPv4 capable and will simply be
configured with an IPv4 [RFC1918]-style address within the home
network and access the IPv4 Internet the same way as the legacy IPv4-
only devices within the home.
Pure IPv6-only devices (i.e., devices that do not include an IPv4
stack) are outside of the scope of this document.
4.3. Directly Connected Device
In broadband home networks, some devices are directly connected to
the broadband service provider. They are connected straight to a
modem, without a home gateway. Those devices are, in fact, acting as
CPEs.
Under this scenario, the customer device is a dual-stack-capable host
that is provisioned by the service provider with IPv6 only. The
device itself acts as a B4 element, and the IPv4 service is provided
by an IPv4-in-IPv6 tunnel, just as in the home gateway/CPE case.
That device can run any combinations of IPv4 and/or IPv6
applications.
A directly connected DS-Lite device SHOULD send its DNS requests over
IPv6 to the IPv6 DNS server it has been configured to use.
Similarly to the previous sections, IPv6 packets follow IPv6 routing,
they do not go through the tunnel, and they are not subject to any
translation.
The support of IPv4-only devices and IPv6-only devices in this
scenario is out of scope for this document.
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5. B4 Element
5.1. Definition
The B4 element is a function implemented on a dual-stack-capable
node, either a directly connected device or a CPE, that creates a
tunnel to an AFTR.
5.2. Encapsulation
The tunnel is a multipoint-to-point IPv4-in-IPv6 tunnel ending on a
service provider AFTR.
See Section 7.1 for additional tunneling considerations.
Note: At this point, DS-Lite only defines IPv4-in-IPv6 tunnels;
however, other types of encapsulation could be defined in the future.
5.3. Fragmentation and Reassembly
Using an encapsulation (IPv4-in-IPv6 or anything else) to carry IPv4
traffic over IPv6 will reduce the effective MTU of the datagram.
Unfortunately, path MTU discovery [RFC1191] is not a reliable method
to deal with this problem.
A solution to deal with this problem is for the service provider to
increase the MTU size of all the links between the B4 element and the
AFTR elements by at least 40 bytes to accommodate both the IPv6
encapsulation header and the IPv4 datagram without fragmenting the
IPv6 packet.
However, as not all service providers will be able to increase their
link MTU, the B4 element MUST perform fragmentation and reassembly if
the outgoing link MTU cannot accommodate the extra IPv6 header. The
original IPv4 packet is not oversized. The packet is oversized after
the IPv6 encapsulation. The inner IPv4 packet MUST NOT be
fragmented. Fragmentation MUST happen after the encapsulation of the
IPv6 packet. Reassembly MUST happen before the decapsulation of the
IPv4 packet. A detailed procedure has been specified in [RFC2473]
Section 7.2.
5.4. AFTR Discovery
In order to configure the IPv4-in-IPv6 tunnel, the B4 element needs
the IPv6 address of the AFTR element. This IPv6 address can be
configured using a variety of methods, ranging from an out-of-band
mechanism, manual configuration, or a variety of DHCPv6 options.
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In order to guarantee interoperability, a B4 element SHOULD implement
the DHCPv6 option defined in [RFC6334].
5.5. DNS
A B4 element is only configured from the service provider with IPv6.
As such, it can only learn the address of a DNS recursive server
through DHCPv6 (or other similar method over IPv6). As DHCPv6 only
defines an option to get the IPv6 address of such a DNS recursive
server, the B4 element cannot easily discover the IPv4 address of
such a recursive DNS server, and as such will have to perform all DNS
resolution over IPv6.
The B4 element can pass this IPv6 address to downstream IPv6 nodes,
but not to downstream IPv4 nodes. As such, the B4 element SHOULD
implement a DNS proxy, following the recommendations of [RFC5625].
To support a security-aware resolver behind the B4 element, the DNS
proxy in the B4 element must also be security aware. Details can be
found in [RFC4033] Section 6.
5.6. Interface Initialization
The B4 element can be implemented in a host and CPE in conjunction
with other technologies such as native dual-stack. The host and the
CPE SHOULD select to start only one technology during initialization.
For example, if the CPE selects to start in native dual-stack mode,
it SHOULD NOT initialize the B4 element. This selection process is
out of scope for this document.
5.7. Well-Known IPv4 Address
Any locally unique IPv4 address could be configured on the IPv4-in-
IPv6 tunnel to represent the B4 element. Configuring such an address
is often necessary when the B4 element is sourcing IPv4 datagrams
directly over the tunnel. In order to avoid conflicts with any other
address, IANA has defined a well-known range, 192.0.0.0/29.
192.0.0.0 is the reserved subnet address. 192.0.0.1 is reserved for
the AFTR element, and 192.0.0.2 is reserved for the B4 element. If a
service provider has a special configuration that prevents the B4
element from using 192.0.0.2, the B4 element MAY use any other
addresses within the 192.0.0.0/29 range.
Note: A range of addresses has been reserved for this purpose. The
intent is to accommodate nodes implementing multiple B4 elements.
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6. AFTR Element
6.1. Definition
An AFTR element is the combination of an IPv4-in-IPv6 tunnel endpoint
and an IPv4-IPv4 NAT implemented on the same node.
6.2. Encapsulation
The tunnel is a point-to-multipoint IPv4-in-IPv6 tunnel ending at the
B4 elements.
See Section 7.1 for additional tunneling considerations.
Note: At this point, DS-Lite only defines IPv4-in-IPv6 tunnels;
however, other types of encapsulation could be defined in the future.
6.3. Fragmentation and Reassembly
As noted previously, fragmentation and reassembly need to be taken
care of by the tunnel endpoints. As such, the AFTR MUST perform
fragmentation and reassembly if the underlying link MTU cannot
accommodate the encapsulation overhead. Fragmentation MUST happen
after the encapsulation on the IPv6 packet. Reassembly MUST happen
before the decapsulation of the IPv6 header. A detailed procedure
has been specified in [RFC2473] Section 7.2.
Fragmentation at the Tunnel Entry-Point is a lightweight operation.
In contrast, reassembly at the Tunnel Exit-Point can be expensive.
When the Tunnel Exit-Point receives the first fragmented packet, it
must wait for the second fragmented packet to arrive in order to
reassemble the two fragmented IPv6 packets for decapsulation. This
requires the Tunnel Exit-Point to buffer and keep track of fragmented
packets. Consider that the AFTR is the Tunnel Exit-Point for many
tunnels. If many devices simultaneously source a large number of
fragmented packets through the AFTR to its managed B4 elements, this
will require the AFTR to buffer and consume enormous resources to
keep track of the flows. This reassembly process will significantly
impact the AFTR's performance. However, this impact only happens
when many clients simultaneously source large IPv4 packets. Since we
believe that the majority of the clients will receive large IPv4
packets (such as watching video streams) instead of sourcing large
IPv4 packets (such as sourcing video streams), reassembly is only a
fraction of the overall AFTR's workload.
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When the AFTR's resources are running below a pre-defined threshold,
the AFTR SHOULD generate a notification to the administrator before
the resources are completely exhausted. The threshold and
notification procedures are implementation dependent and are out of
scope for this document.
Methods to avoid fragmentation, such as rewriting the TCP Maximum
Segment Size (MSS) option or using technologies such as the
Subnetwork Encapsulation and Adaptation Layer as defined in
[RFC5320], are out of scope for this document.
6.4. DNS
As noted previously, a DS-Lite node implementing a B4 element will
perform DNS resolution over IPv6. As a result, DNS packets are not
expected to go through the AFTR element.
6.5. Well-Known IPv4 Address
The AFTR SHOULD use the well-known IPv4 address 192.0.0.1 reserved by
IANA to configure the IPv4-in-IPv6 tunnel. That address can then be
used to report ICMP problems and will appear in traceroute outputs.
6.6. Extended Binding Table
The NAT binding table of the AFTR element is extended to include the
source IPv6 address of the incoming packets. This IPv6 address is
used to disambiguate between the overlapping IPv4 address space of
the service provider customers.
By doing a reverse lookup in the extended IPv4 NAT binding table, the
AFTR knows how to reconstruct the IPv6 encapsulation when the packets
come back from the Internet. That way, there is no need to keep a
static configuration for each tunnel.
7. Network Considerations
7.1. Tunneling
Tunneling MUST be done in accordance to [RFC2473] and [RFC4213].
Traffic classes ([RFC2474]) from the IPv4 headers MUST be carried
over to the IPv6 headers and vice versa.
7.2. Multicast Considerations
Discussion of multicast is out of scope for this document.
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8. NAT Considerations
8.1. NAT Pool
The AFTR MAY be provisioned with different NAT pools. The address
ranges in the pools may be disjoint but MUST NOT be overlapped.
Operators may implement policies in the AFTR to assign clients in
different pools. For example, an AFTR can have two interfaces. Each
interface will have a disjoint pool NAT assigned to it. In another
case, a policy implemented on the AFTR may specify that one set of
B4s will use NAT pool 1 and a different set of B4s will use NAT
pool 2.
8.2. NAT Conformance
A Dual-Stack Lite AFTR MUST implement behavior conforming to the best
current practice, currently documented in [RFC4787], [RFC5508], and
[RFC5382]. More discussions about carrier-grade NATs can be found in
[LSN-REQS].
8.3. Application Level Gateways (ALGs)
The AFTR performs NAT-44 and inherits the limitations of NAT. Some
protocols require ALGs in the NAT device to traverse through the NAT.
For example, Active FTP requires the ALG to work properly. ALGs
consume resources, and there are many different types of ALGs. The
AFTR is a shared network device that supports a large number of B4
elements. It is impossible for the AFTR to implement every current
and future ALG.
8.4. Sharing Global IPv4 Addresses
The AFTR shares a single IP with multiple users. This helps to
increase the IPv4 address utilization. However, it also brings some
issues such as logging and lawful intercept. More considerations on
sharing the port space of IPv4 addresses can be found in [RFC6269].
8.5. Port Forwarding / Keep Alive
The PCP working group is standardizing a control plane to the
carrier-grade NAT [LSN-REQS] in the IETF. The Port Control Protocol
(PCP) enables applications to directly negotiate with the NAT to open
ports and negotiate lifetime values to avoid keep-alive traffic.
More on PCP can be found in [PCP-BASE].
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9. Acknowledgements
The authors would like to acknowledge the role of Mark Townsley for
his input on the overall architecture of this technology by pointing
this work in the direction of [SNAT]. Note that this document
results from a merging of [DURAND-DS-LITE] and [SNAT]. Also to be
acknowledged are the many discussions with a number of people
including Shin Miyakawa, Katsuyasu Toyama, Akihide Hiura, Takashi
Uematsu, Tetsutaro Hara, Yasunori Matsubayashi, and Ichiro Mizukoshi.
The authors would also like to thank David Ward, Jari Arkko, Thomas
Narten, and Geoff Huston for their constructive feedback. Special
thanks go to Dave Thaler and Dan Wing for their reviews and comments.
10. IANA Considerations
Per this document, IANA has allocated a well-known IPv4 192.0.0.0/29
network prefix. That range is used to number the Dual-Stack Lite
interfaces. Reserving a /29 allows for 6 possible interfaces on a
multi-home node. The IPv4 address 192.0.0.1 is reserved as the IPv4
address of the default router for such Dual-Stack Lite hosts.
11. Security Considerations
Security issues associated with NAT have long been documented. See
[RFC2663] and [RFC2993].
However, moving the NAT functionality from the CPE to the core of the
service provider network and sharing IPv4 addresses among customers
create additional requirements when logging data for abuse usage.
With any architecture where an IPv4 address does not uniquely
represent an end host, IPv4 addresses and timestamps are no longer
sufficient to identify a particular broadband customer. The AFTR
should have the capability to log the tunnel-id, protocol, ports/IP
addresses, and the creation time of the NAT binding to uniquely
identify the user sessions. Exact details of what is logged are
implementation specific and out of scope for this document.
The AFTR performs translation functions for interior IPv4 hosts using
RFC 1918 addresses or the IANA reserved address range (192.0.0.0/29).
In some circumstances, an ISP may provision policies in the AFTR and
instruct the AFTR to bypass translation functions based on . When the AFTR receives a packet
with matching information of the policy from the interior host, the
AFTR can simply forward the packet without translation. The
addresses, ports, and protocol information must be provisioned on the
AFTR before receiving the packet. The provisioning mechanism is out
of scope for this specification.
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When decapsulating packets, the AFTR MUST only forward packets
sourced by RFC 1918 addresses, an IANA reserved address range, or any
other out-of-band pre-authorized addresses. The AFTR MUST drop all
other packets. This prevents rogue devices from launching denial-of-
service attacks using unauthorized public IPv4 addresses in the IPv4
source header field or an unauthorized transport port range in the
IPv4 transport header field. For example, rogue devices could
bombard a public web server by launching a TCP SYN ACK attack
[RFC4987]. The victim will receive TCP SYN from random IPv4 source
addresses at a rapid rate and deny TCP services to legitimate users.
With IPv4 addresses shared by multiple users, ports become a critical
resource. As such, some mechanisms need to be put in place by an
AFTR to limit port usage, either by rate-limiting new connections or
putting a hard limit on the maximum number of ports usable by a
single user. If this number is high enough, it should not interfere
with normal usage and still provide reasonable protection of the
shared pool. More considerations on sharing IPv4 addresses can be
found in [RFC6269]. Other considerations and recommendations on
logging can be found in [RFC6302].
AFTRs should support ways to limit service only to registered
customers. One simple option is to implement an IPv6 ingress filter
on the AFTR's tunnel interface to accept only the IPv6 address range
defined in the filter.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition
Mechanisms for IPv6 Hosts and Routers", RFC 4213,
October 2005.
Durand, et al. Standards Track [Page 13]
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[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, August 2009.
[RFC6334] Hankins, D. and T. Mrugalski, "Dynamic Host Configuration
Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite",
RFC 6334, August 2011.
12.2. Informative References
[DURAND-DS-LITE]
Durand, A., "Dual-stack lite broadband deployments post
IPv4 exhaustion", Work in Progress, July 2008.
[LSN-REQS] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
A., and H. Ashida, "Common requirements for Carrier Grade
NAT (CGN)", Work in Progress, July 2011.
[PCP-BASE] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R.,
and P. Selkirk, "Port Control Protocol (PCP)", Work
in Progress, July 2011.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4787] Audet, F., Ed., and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, January 2007.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and
Adaptation Layer (SEAL)", RFC 5320, February 2010.
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[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and
P. Srisuresh, "NAT Behavioral Requirements for TCP",
BCP 142, RFC 5382, October 2008.
[RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
April 2009.
[RFC5571] Storer, B., Pignataro, C., Ed., Dos Santos, M., Stevant,
B., Ed., Toutain, L., and J. Tremblay, "Softwire Hub and
Spoke Deployment Framework with Layer Two Tunneling
Protocol Version 2 (L2TPv2)", RFC 5571, June 2009.
[RFC6269] Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing", RFC 6269,
June 2011.
[RFC6302] Durand, A., Gashinsky, I., Lee, D., and S. Sheppard,
"Logging Recommendations for Internet-Facing Servers",
BCP 162, RFC 6302, June 2011.
[SNAT] Droms, R. and B. Haberman, "Softwires Network Address
Translation (SNAT)", Work in Progress, July 2008.
Durand, et al. Standards Track [Page 15]
RFC 6333 Dual-Stack Lite August 2011
Appendix A. Deployment Considerations
A.1. AFTR Service Distribution and Horizontal Scaling
One of the key benefits of the Dual-Stack Lite technology lies in the
fact that it is a tunnel-based solution. As such, tunnel endpoints
can be anywhere in the service provider network.
Using the DHCPv6 tunnel endpoint option [RFC6334], service providers
can create groups of users sharing the same AFTR. Those groups can
be merged or divided at will. This leads to a horizontally scaled
solution, where more capacity is added with more AFTRs. As those
groups of users can evolve over time, it is best to make sure that
AFTRs do not require per-user configuration in order to provide
service.
A.2. Horizontal Scaling
A service provider can start using just a few centralized AFTRs.
Later, when more capacity is needed, more AFTRs can be added and
pushed closer to the edges of the access network.
A.3. High Availability
An important element in the design of the Dual-Stack Lite technology
is the simplicity of implementation on the customer side. An IP4-in-
IPv6 tunnel and a default route over it in the B4 element are all
that is needed to get IPv4 connectivity. It is assumed that high
availability is the responsibility of the service provider, not the
customer devices implementing Dual-Stack Lite. As such, a single
IPv6 address of the tunnel endpoint is provided in the DHCPv6 option
defined in [RFC6334]. Specific means to achieve high availability on
the service provider side are outside the scope of this
specification.
A.4. Logging
DS-Lite AFTR implementation should offer the functionality to log NAT
binding creations or other ways to keep track of the ports/IP
addresses used by customers. This is both to support
troubleshooting, which is very important to service providers trying
to figure out why something may not be working, and to meet region-
specific requirements for responding to legally binding requests for
information from law enforcement authorities.
Durand, et al. Standards Track [Page 16]
RFC 6333 Dual-Stack Lite August 2011
Appendix B. Examples
B.1. Gateway-Based Architecture
This architecture is targeted at residential broadband deployments
but can be adapted easily to other types of deployment where the
installed base of IPv4-only devices is important.
Consider a scenario where a Dual-Stack Lite CPE is provisioned only
with IPv6 in the WAN port, not IPv4. The CPE acts as an IPv4 DHCP
server for the LAN (wireline and wireless) handing out [RFC1918]
addresses. In addition, the CPE may support IPv6 Auto-Configuration
and/or a DHCPv6 server for the LAN. When an IPv4-only device
connects to the CPE, that CPE will hand out a [RFC1918] address to
the device. When a dual-stack-capable device connects to the CPE,
that CPE will hand out a [RFC1918] address and a global IPv6 address
to the device. Besides, the CPE will create an IPv4-in-IPv6 softwire
tunnel [RFC5571] to an AFTR that resides in the service provider
network.
When the device accesses IPv6 service, it will send the IPv6 datagram
to the CPE natively. The CPE will route the traffic upstream to the
IPv6 default gateway.
When the device accesses IPv4 service, it will source the IPv4
datagram with the [RFC1918] address and send the IPv4 datagram to the
CPE. The CPE will encapsulate the IPv4 datagram inside the IPv4-in-
IPv6 softwire tunnel and forward the IPv6 datagram to the AFTR. This
is in contrast to what the CPE normally does today, which is to NAT
the [RFC1918] address to the public IPv4 address and route the
datagram upstream. When the AFTR receives the IPv6 datagram, it will
decapsulate the IPv6 header and perform an IPv4-to-IPv4 NAT on the
source address.
As illustrated in Figure 1, this Dual-Stack Lite deployment model
consists of three components: the Dual-Stack Lite home router with a
B4 element, the AFTR, and a softwire between the B4 element acting as
softwire initiator (SI) [RFC5571] in the Dual-Stack Lite home router
and the softwire concentrator (SC) [RFC5571] in the AFTR. The AFTR
performs IPv4-IPv4 NAT translations to multiplex multiple subscribers
through a pool of global IPv4 addresses. Overlapping address spaces
used by subscribers are disambiguated through the identification of
tunnel endpoints.
Durand, et al. Standards Track [Page 17]
RFC 6333 Dual-Stack Lite August 2011
+-----------+
| Host |
+-----+-----+
|10.0.0.1
|
|
|10.0.0.2
+---------|---------+
| | |
| Home router |
|+--------+--------+|
|| B4 ||
|+--------+--------+|
+--------|||--------+
|||2001:db8:0:1::1
|||
|||
RFC 6333 Dual-Stack Lite August 2011
Notes:
o The Dual-Stack Lite home router is not required to be on the same
link as the host.
o The Dual-Stack Lite home router could be replaced by a Dual-Stack
Lite router in the service provider network.
The resulting solution accepts an IPv4 datagram that is translated
into an IPv4-in-IPv6 softwire datagram for transmission across the
softwire. At the corresponding endpoint, the IPv4 datagram is
decapsulated, and the translated IPv4 address is inserted based on a
translation from the softwire.
B.1.1. Example Message Flow
In the example shown in Figure 2, the translation tables in the AFTR
are configured to forward between IP/TCP (10.0.0.1/10000) and IP/TCP
(192.0.2.1/5000). That is, a datagram received by the Dual-Stack
Lite home router from the host at address 10.0.0.1, using TCP DST
port 10000, will be translated to a datagram with IPv4 SRC address
192.0.2.1 and TCP SRC port 5000 in the Internet.
Durand, et al. Standards Track [Page 19]
RFC 6333 Dual-Stack Lite August 2011
+-----------+
| Host |
+-----+-----+
| |10.0.0.1
IPv4 datagram 1 | |
| |
v |10.0.0.2
+---------|---------+
| | |
| home router |
|+--------+--------+|
|| B4 ||
|+--------+--------+|
+--------|||--------+
| |||2001:db8:0:1::1
IPv6 datagram 2| |||
| |||
RFC 6333 Dual-Stack Lite August 2011
+-----------------+--------------+-----------------+
| Datagram | Header field | Contents |
+-----------------+--------------+-----------------+
| IPv4 datagram 1 | IPv4 Dst | 198.51.100.1 |
| | IPv4 Src | 10.0.0.1 |
| | TCP Dst | 80 |
| | TCP Src | 10000 |
| --------------- | ------------ | ------------- |
| IPv6 datagram 2 | IPv6 Dst | 2001:db8:0:2::1 |
| | IPv6 Src | 2001:db8:0:1::1 |
| | IPv4 Dst | 198.51.100.1 |
| | IPv4 Src | 10.0.0.1 |
| | TCP Dst | 80 |
| | TCP Src | 10000 |
| --------------- | ------------ | ------------- |
| IPv4 datagram 3 | IPv4 Dst | 198.51.100.1 |
| | IPv4 Src | 192.0.2.1 |
| | TCP Dst | 80 |
| | TCP Src | 5000 |
+-----------------+--------------+-----------------+
Datagram Header Contents
When datagram 1 is received by the Dual-Stack Lite home router, the
B4 element encapsulates the datagram in datagram 2 and forwards it to
the Dual-Stack Lite carrier-grade NAT over the softwire.
When the tunnel concentrator in the AFTR receives datagram 2, it
forwards the IPv4 datagram to the NAT, which determines from its NAT
table that the datagram received on the softwire with TCP SRC
port 10000 should be translated to datagram 3 with IPv4 SRC address
192.0.2.1 and TCP SRC port 5000.
Figure 3 shows an inbound message received at the AFTR. When the NAT
function in the AFTR receives datagram 1, it looks up the IP/TCP DST
information in its translation table. In the example in Figure 3,
the NAT changes the TCP DST port to 10000, sets the IP DST address to
10.0.0.1, and forwards the datagram to the softwire. The B4 in the
home router decapsulates the IPv4 datagram from the inbound softwire
datagram and forwards it to the host.
Durand, et al. Standards Track [Page 21]
RFC 6333 Dual-Stack Lite August 2011
+-----------+
| Host |
+-----+-----+
^ |10.0.0.1
IPv4 datagram 3 | |
| |
| |10.0.0.2
+---------|---------+
| +-+-+ |
| home router |
|+--------+--------+|
|| B4 ||
|+--------+--------+|
+--------|||--------+
^ |||2001:db8:0:1::1
IPv6 datagram 2 | |||
| |||
RFC 6333 Dual-Stack Lite August 2011
+-----------------+--------------+-----------------+
| Datagram | Header field | Contents |
+-----------------+--------------+-----------------+
| IPv4 datagram 1 | IPv4 Dst | 192.0.2.1 |
| | IPv4 Src | 198.51.100.1 |
| | TCP Dst | 5000 |
| | TCP Src | 80 |
| --------------- | ------------ | ------------- |
| IPv6 datagram 2 | IPv6 Dst | 2001:db8:0:1::1 |
| | IPv6 Src | 2001:db8:0:2::1 |
| | IPv4 Dst | 10.0.0.1 |
| | IPv4 Src | 198.51.100.1 |
| | TCP Dst | 10000 |
| | TCP Src | 80 |
| --------------- | ------------ | ------------- |
| IPv4 datagram 3 | IPv4 Dst | 10.0.0.1 |
| | IPv4 Src | 198.51.100.1 |
| | TCP Dst | 10000 |
| | TCP Src | 80 |
+-----------------+--------------+-----------------+
Datagram Header Contents
B.1.2. Translation Details
The AFTR has a NAT that translates between softwire/port pairs and
IPv4-address/port pairs. The same translation is applied to IPv4
datagrams received on the device's external interface and from the
softwire endpoint in the device.
In Figure 2, the translator network interface in the AFTR is on the
Internet, and the softwire interface connects to the Dual-Stack Lite
home router. The AFTR translator is configured as follows:
Network interface: Translate IPv4 destination address and TCP
destination port to the softwire identifier and TCP destination
port
Softwire interface: Translate softwire identifier and TCP source
port to IPv4 source address and TCP source port
Here is how the translation in Figure 3 works:
o Datagram 1 is received on the AFTR translator network interface.
The translator looks up the IPv4-address/port pair in its
translator table, rewrites the IPv4 destination address to
10.0.0.1 and the TCP source port to 10000, and forwards the
datagram to the softwire.
Durand, et al. Standards Track [Page 23]
RFC 6333 Dual-Stack Lite August 2011
o The IPv4 datagram is received on the Dual-Stack Lite home router
B4. The B4 function extracts the IPv4 datagram, and the Dual-
Stack Lite home router forwards datagram 3 to the host.
+------------------------------------+--------------------+
| Softwire-Id/IPv4/Prot/Port | IPv4/Prot/Port |
+------------------------------------+--------------------+
| 2001:db8:0:1::1/10.0.0.1/TCP/10000 | 192.0.2.1/TCP/5000 |
+------------------------------------+--------------------+
Dual-Stack Lite Carrier-Grade NAT Translation Table
The Softwire-Id is the IPv6 address assigned to the Dual-Stack Lite
CPE. Hosts behind the same Dual-Stack Lite home router have the same
Softwire-Id. The source IPv4 address is the [RFC1918] address
assigned by the Dual-Stack home router and is unique to each host
behind the CPE. The AFTR would receive packets sourced from
different IPv4 addresses in the same softwire tunnel. The AFTR
combines the Softwire-Id and IPv4 address/port [Softwire-Id, IPv4+
Port] to uniquely identify the host behind the same Dual-Stack Lite
home router.
B.2. Host-Based Architecture
This architecture is targeted at new, large-scale deployments of
dual-stack-capable devices implementing a Dual-Stack Lite interface.
Consider a scenario where a Dual-Stack Lite host device is directly
connected to the service provider network. The host device is dual-
stack capable but only provisioned with an IPv6 global address.
Besides, the host device will pre-configure a well-known IPv4
non-routable address; see Section 10 (IANA Considerations). This
well-known IPv4 non-routable address is similar to the 127.0.0.1
loopback address. Every host device that implements Dual-Stack Lite
will pre-configure the same address. This address will be used to
source the IPv4 datagram when the device accesses IPv4 services.
Besides, the host device will create an IPv4-in-IPv6 softwire tunnel
to an AFTR. The carrier-grade NAT will reside in the service
provider network.
When the device accesses IPv6 service, the device will send the IPv6
datagram natively to the default gateway.
Durand, et al. Standards Track [Page 24]
RFC 6333 Dual-Stack Lite August 2011
When the device accesses IPv4 service, it will source the IPv4
datagram with the well-known non-routable IPv4 address. Then, the
host device will encapsulate the IPv4 datagram inside the IPv4-in-
IPv6 softwire tunnel and send the IPv6 datagram to the AFTR. When
the AFTR receives the IPv6 datagram, it will decapsulate the IPv6
header and perform IPv4-to-IPv4 NAT on the source address.
This scenario works on both wireline and wireless networks. A
typical wireless device will connect directly to the service provider
without a CPE in between.
As illustrated in Figure 4, this Dual-Stack Lite deployment model
consists of three components: the Dual-Stack Lite host, the AFTR, and
a softwire between the softwire initiator B4 in the host and the
softwire concentrator in the AFTR. The Dual-Stack Lite host is
assumed to have IPv6 service and can exchange IPv6 traffic with the
AFTR.
The AFTR performs IPv4-IPv4 NAT translations to multiplex multiple
subscribers through a pool of global IPv4 addresses. Overlapping
IPv4 address spaces used by the Dual-Stack Lite hosts are
disambiguated through the identification of tunnel endpoints.
In this situation, the Dual-Stack Lite host configures the IPv4
address 192.0.0.2 out of the well-known range 192.0.0.0/29 (defined
by IANA) on its B4 interface. It also configures the first
non-reserved IPv4 address of the reserved range, 192.0.0.1, as the
address of its default gateway.
Durand, et al. Standards Track [Page 25]
RFC 6333 Dual-Stack Lite August 2011
+-------------------+
| |
| Host 192.0.0.2 |
|+--------+--------+|
|| B4 ||
|+--------+--------+|
+--------|||--------+
|||2001:db8:0:1::1
|||
|||
RFC 6333 Dual-Stack Lite August 2011
B.2.1. Example Message Flow
In the example shown in Figure 5, the translation tables in the AFTR
are configured to forward between IP/TCP (192.0.0.2/10000) and IP/TCP
(192.0.2.1/5000). That is, a datagram received from the host at
address 192.0.0.2, using TCP DST port 10000, will be translated to a
datagram with IPv4 SRC address 192.0.2.1 and TCP SRC port 5000 in the
Internet.
Durand, et al. Standards Track [Page 27]
RFC 6333 Dual-Stack Lite August 2011
+-------------------+
| |
|Host 192.0.0.2 |
|+--------+--------+|
|| B4 ||
|+--------+--------+|
+--------|||--------+
| |||2001:db8:0:1::1
IPv6 datagram 1| |||
| |||
RFC 6333 Dual-Stack Lite August 2011
+-----------------+--------------+-----------------+
| Datagram | Header field | Contents |
+-----------------+--------------+-----------------+
| IPv6 datagram 1 | IPv6 Dst | 2001:db8:0:2::1 |
| | IPv6 Src | 2001:db8:0:1::1 |
| | IPv4 Dst | 198.51.100.1 |
| | IPv4 Src | 192.0.0.2 |
| | TCP Dst | 80 |
| | TCP Src | 10000 |
| --------------- | ------------ | ------------- |
| IPv4 datagram 2 | IPv4 Dst | 198.51.100.1 |
| | IPv4 Src | 192.0.2.1 |
| | TCP Dst | 80 |
| | TCP Src | 5000 |
+-----------------+--------------+-----------------+
Datagram Header Contents
When sending an IPv4 packet, the Dual-Stack Lite host encapsulates it
in datagram 1 and forwards it to the AFTR over the softwire.
When it receives datagram 1, the concentrator in the AFTR hands the
IPv4 datagram to the NAT, which determines from its translation table
that the datagram received on the softwire with TCP SRC port 10000
should be translated to datagram 3 with IPv4 SRC address 192.0.2.1
and TCP SRC port 5000.
Figure 6 shows an inbound message received at the AFTR. When the NAT
function in the AFTR receives datagram 1, it looks up the IP/TCP DST
in its translation table. In the example in Figure 6, the NAT
translates the TCP DST port to 10000, sets the IP DST address to
192.0.0.2, and forwards the datagram to the softwire. The B4 inside
the host decapsulates the IPv4 datagram from the inbound softwire
datagram, and forwards it to the host's application layer.
Durand, et al. Standards Track [Page 29]
RFC 6333 Dual-Stack Lite August 2011
+-------------------+
| |
|Host 192.0.0.2 |
|+--------+--------+|
|| B4 ||
|+--------+--------+|
+--------|||--------+
^ |||2001:db8:0:1::1
IPv6 datagram 2 | |||
| |||
RFC 6333 Dual-Stack Lite August 2011
+-----------------+--------------+-----------------+
| Datagram | Header field | Contents |
+-----------------+--------------+-----------------+
| IPv4 datagram 1 | IPv4 Dst | 192.0.2.1 |
| | IPv4 Src | 198.51.100.1 |
| | TCP Dst | 5000 |
| | TCP Src | 80 |
| --------------- | ------------ | ------------- |
| IPv6 datagram 2 | IPv6 Dst | 2001:db8:0:1::1 |
| | IPv6 Src | 2001:db8:0:2::1 |
| | IPv4 Dst | 192.0.0.2 |
| | IPv4 Src | 198.51.100.1 |
| | TCP Dst | 10000 |
| | TCP Src | 80 |
+-----------------+--------------+-----------------+
Datagram Header Contents
B.2.2. Translation Details
The AFTR translation steps are the same as in Appendix B.1.2. One
difference is that all the host-based B4s will use the same well-
known IPv4 address 192.0.0.2. To uniquely identify the host-based
B4, the AFTR will use the host-based B4's IPv6 address, which is
unique for the host.
+-------------------------------------+--------------------+
| Softwire-Id/IPv4/Prot/Port | IPv4/Prot/Port |
+-------------------------------------+--------------------+
| 2001:db8:0:1::1/192.0.0.2/TCP/10000 | 192.0.2.1/TCP/5000 |
+-------------------------------------+--------------------+
Dual-Stack Lite Carrier-Grade NAT Translation Table
The Softwire-Id is the IPv6 address assigned to the Dual-Stack host.
Each host has a unique Softwire-Id. The source IPv4 address is one
of the well-known IPv4 addresses. The AFTR could receive packets
from different hosts sourced from the same IPv4 well-known address
from different softwire tunnels. Similar to the gateway
architecture, the AFTR combines the Softwire-Id and IPv4 address/port
[Softwire-Id, IPv4+Port] to uniquely identify the individual host.
Durand, et al. Standards Track [Page 31]
RFC 6333 Dual-Stack Lite August 2011
Authors' Addresses
Alain Durand
Juniper Networks
1194 North Mathilda Avenue
Sunnyvale, CA 94089-1206
USA
EMail: adurand@juniper.net
Ralph Droms
Cisco
1414 Massachusetts Avenue
Boxborough, MA 01714
USA
EMail: rdroms@cisco.com
James Woodyatt
Apple
1 Infinite Loop
Cupertino, CA 95014
USA
EMail: jhw@apple.com
Yiu L. Lee
Comcast
One Comcast Center
Philadelphia, PA 19103
USA
EMail: yiu_lee@cable.comcast.com
Durand, et al. Standards Track [Page 32]